The SI joint, or sacroiliac joint, is the connection between your spine and your pelvis. You have two of them, one on each side, and they sit right where the triangular bone at the base of your spine (the sacrum) meets the large wing-shaped bones of your pelvis (the ilia). These joints are responsible for transferring the entire weight of your upper body into your legs every time you stand, walk, or run. They’re also a surprisingly common source of low back pain, accounting for roughly 10% to 25% of chronic mechanical low back pain cases.
Where the SI Joint Sits and How It’s Built
The SI joint is tucked deep beneath muscle and thick ligament on either side of your lower back, just below the beltline. If you place your thumbs on the two bony bumps at the top of your buttocks, you’re pressing almost directly over them. Unlike your knee or shoulder, the SI joint isn’t designed for large movements. It’s a hybrid joint: part of it has smooth cartilage and lubricating fluid like a typical moveable joint, while the upper portion is bound together by dense fibrous tissue more like the joints between your skull bones.
What holds it all together is an extensive network of ligaments, some of the strongest in the body. Bands of tissue run across the front, back, and underside of the joint, locking it in place during weight-bearing. Two particularly important ligaments on the back of the pelvis resist the tendency of the sacrum to slide forward and downward when you’re standing or carrying a load. This architecture prioritizes stability over mobility, which makes sense given that the joint’s primary job is to absorb and redirect force rather than produce movement.
How the SI Joint Moves
The SI joint moves, but barely. Total rotation is only about 1.5 degrees, and the sliding motion it allows is limited to roughly 2 to 4 millimeters. That small range of movement serves a critical purpose: it functions as a shock absorber between your spine and legs. When you land from a jump or take a heavy step, the sacrum tips slightly forward under the load. Your body then pushes back, lifting the joint against gravity. This tiny rocking motion dissipates force that would otherwise travel straight up your spine.
The joint also converts rotational force from your legs into usable energy for the rest of your body. When you walk, your pelvis rotates slightly with each stride. The SI joints manage that torque, passing it upward in a controlled way. Without them, every step would send a jolt through your lower back.
What SI Joint Pain Feels Like
SI joint pain typically shows up as a deep ache on one side of the lower back, right over the joint itself. The classic location is a narrow vertical strip of pain, roughly 3 centimeters wide and 10 centimeters long, running downward from the bony bump at the top of your buttock. Pain often radiates into the buttock and sometimes down the back of the thigh, which is why it’s frequently mistaken for sciatica or a herniated disc.
The pain tends to worsen with activities that load one side of the pelvis more than the other: climbing stairs, standing on one leg, rolling over in bed, or getting out of a car. Sitting for long periods can aggravate it too, especially on hard surfaces. Some people notice it most during transitions, like going from sitting to standing, rather than during sustained activity.
Common Causes of SI Joint Problems
SI joint dysfunction can develop from several directions. The most common is mechanical stress from everyday wear. Osteoarthritis can break down the cartilage inside the joint over time, especially in people over 50. A leg length difference, even a small one, changes how force distributes through the pelvis and can overload one SI joint for years before symptoms appear. Prior lumbar spine surgery, particularly fusion of the lowest vertebrae, shifts extra mechanical demand onto the SI joints below.
Pregnancy is another well-known trigger. Hormonal changes loosen the ligaments around the SI joint to prepare the pelvis for childbirth, and the combination of increased joint laxity, weight gain, and altered posture can create significant pain. For most women this resolves after delivery, but for some it persists.
Inflammatory conditions also target the SI joint. Ankylosing spondylitis, a type of arthritis that primarily affects the spine, often starts with inflammation in the SI joints before progressing upward. Psoriatic arthritis and inflammatory bowel diseases like Crohn’s disease and ulcerative colitis also increase the risk. In these cases, SI joint inflammation (called sacroiliitis) is driven by the immune system rather than by mechanical wear.
How SI Joint Dysfunction Is Diagnosed
Diagnosing the SI joint as the source of low back pain is notoriously tricky. Imaging alone is often unreliable because age-related changes in the joint don’t always correlate with pain. Instead, clinicians rely on a combination of hands-on provocative tests that stress the joint in specific ways to reproduce the patient’s familiar pain.
The most useful of these tests include the thigh thrust, where pressure is applied through the bent knee to load the SI joint, and the FABER test, which places the hip in a figure-four position and pushes down on the knee. The thigh thrust has the best sensitivity at about 74%, meaning it catches most true SI joint problems. The FABER test is better at ruling out the SI joint, with a specificity around 67%. No single test is definitive on its own, so most providers use a cluster of three or more. If the majority reproduce pain, the SI joint becomes a strong suspect.
The gold standard for confirmation is a diagnostic injection. A small amount of numbing medication is placed directly into the joint under imaging guidance. If the pain temporarily disappears, the SI joint is confirmed as the source.
Treatment Options
Most SI joint pain responds to conservative care. Physical therapy is the foundation, focusing on strengthening the muscles that stabilize the pelvis. The gluteal muscles on the outside of the hip are a primary target, particularly through exercises like sidelying leg lifts. The inner thigh muscles also play a role in pelvic alignment; a simple standing ball squeeze can help activate them. Stretching the small rotator muscles deep in the buttock can relieve pressure on the sciatic nerve when it’s being irritated by the dysfunction. An SI belt, a tight strap worn around the pelvis, can provide external compression and immediate relief during flare-ups.
When physical therapy and over-the-counter pain relief aren’t enough, corticosteroid injections into the joint are a common next step. Research spanning a decade of practice shows that a single injection reduces pain scores significantly at one month, with meaningful relief still present at six months and, to a lesser degree, at one year. Repeat injections produce similar short-term results, though the benefit at the one-year mark tends to be smaller.
For people who get consistent but temporary relief from injections and haven’t improved with therapy, SI joint fusion is an option. This minimally invasive procedure permanently stabilizes the joint with small implants placed through a small incision. More than 4 out of 5 people who undergo the surgery report a successful outcome, with pain reduced by at least half even years later.
Why the SI Joint Gets Overlooked
Despite being responsible for up to a quarter of chronic low back pain cases, the SI joint is one of the most frequently missed diagnoses in spine care. Its pain pattern overlaps heavily with lumbar disc problems and hip conditions. It doesn’t show up reliably on MRI unless there’s active inflammation. And because it moves so little, many providers don’t think to test it. If you’ve had persistent one-sided low back and buttock pain that hasn’t responded to treatments aimed at your lumbar spine, the SI joint is worth investigating.